1. Field of the Invention
The present invention relates to a matrix wiring substrate used as for example a drive electrode assembly of an active matrix liquid crystal display, and more specifically to a matrix wiring substrate having a common electrode for preventing an electrostatic breakdown or destruction in a manufacturing process.
2. Description of Related Art
In the prior art, this type of matrix wiring substrate has been constructed by stacking on a substrate a lower level wiring and an upper level wiring, interposing therebetween an insulating layer or a combination of an insulating layer and a semiconductor layer. In this structure, the insulating layer is apt to be broken down or destroyed by static electricity, so that the lower level wiring and the upper level wiring are short-circuited. Therefore, various countermeasures have been proposed in order to prevent the electrostatic breakdown or destruction.
Referring to FIG. 1, there is shown a circuit diagram of one typical conventional matrix wiring substrate provided with the means for preventing the electrostatic destruction. The shown example is a matrix wiring substrate of a TFT (thin film transistor) active matrix liquid crystal display.
A number of pixel electrodes 1 corresponding to pixels of the liquid crystal display and each associated with one TFT 2 for individually driving the corresponding pixel electrode 1 are arranged on a glass substrate 1 in the form of a matrix. The TFTs 2 are formed of amorphous silicon (a-Si). Gates of all the TFTs 2 included in each one row are connected to a corresponding gate bus line 3 extending in an X direction, and drains of all the TFTs 2 included in each one column are connected to a corresponding drain bus line 4 extending in a Y direction. Each of the gate bus lines 2 is connected at its one end to a gate terminal 5, and at its other end to a checking pad 6. In addition, each of the drain bus line 4 is connected at its one end to a drain terminal 7, and at its other end to a checking pad 8.
For example, the gate bus lines 3 are formed from a lower level wiring, and the drain bus line 4 are formed from a upper level wiring. Therefore, crossing points P between the gate bus lines 3 and the drain bus line 4 occurs in the same number as the number of the pixels 1. In addition, the TFTs 2 provided one for each of the pixels 1 have a gate insulator sandwiched between the upper level wiring and the lower level wiring, similarly to a MOS transistor.
With the above mentioned arrangement, the crossing points P between the upper and lower level wirings and the TFTs 2 are liable to be electrostatically broken in a manufacturing process. This has become one of causes of lowering a yield of production.
In order to prevent the electrostatic breakdown or destruction, common electrodes 9 formed from a lower level wiring are provided at an outside of the gate terminals 5 and the checking electrodes, and common electrodes 10 formed from a upper level wiring are provided at an outside of the drain terminals 7 and the checking electrodes 8, and these common electrodes 9 and 10 are interconnected at four corners by means of through-hole contacts 11. The gate terminals 5 and the drain terminals 7 are connected to the common electrodes 9 and 10, respectively. With this arrangement, the upper and lower level wirings are maintained at an equal potential, so that occurrence of the electrostatic breakdown or destruction can be effectively prevented.
Incidentally, at a late stage of the manufacturing process, the common electrodes 9 and 10 are cut away by cutting along an imaginary line which connects cut marks 12.
For example, in the liquid crystal display, it is necessary to provide the upper level wirings and the lower level wirings respectively in the number corresponding to a horizontal pixel number determining a horizontal resolution of an image and in the number corresponding to a vertical pixel number determining a vertical resolution of the image. As a result, the number of the wirings exceed 1000. Therefore, from a viewpoint of product control, it is necessary to sufficiently perform a good/defective checking of wirings in the course of the manufacturing process.
In the above mentioned conventional matrix wiring substrate, however, since all the wirings are shunted to the common electrodes for prevention of the electrostatic breakdown or destruction, a sufficient checking sensibility could not have been obtained in a wiring checking (I-V checking) for discriminating the quality of the wirings. As a result, the checking has been very difficult.